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Title: Washboard and Fluted Terrains on Pluto as Evidence for Ancient Glaciation— Short Story:  Strategic Plan Core Capability/Objective Reference: 2.3 (Understand Formation of Planets, Satellites and Rings, and Their Long-Term Evolution and Dynamics).  On September 7th 2018 the journal Nature Astronomy accepted a letter authored by Oliver White of the SETI Institute/NASA ARC, which included Jeff Moore, Tanguy Bertrand, and Kim Ennico as Ames-based co-authors.  The focus of the letter is the distinctive landscapes termed “washboard” and “fluted” terrains, which border the vast nitrogen ice plains of Sputnik Planitia along its northwest margin, and which are amongst the most enigmatic landforms yet seen on Pluto. These terrains consist of parallel to sub-parallel ridges that display a remarkably consistent ENE-WSW orientation, a configuration that does not readily point to a simple analogous terrestrial or planetary process or landform.  The research described in the letter uses mapping and analysis of their morphometry and distribution as a means to determine their origin.  Washboard terrain occurs in level topographic settings within valley floors, basins and uplands, while fluted terrain is seen on steeper spurs, massifs, and crater walls that separate basins and valleys.  Both types are featured in the accompanying figure, in which illumination is from the top.  These terrains occur at the location on Sputnik Planitia’s perimeter where elevations and slopes leading into the surrounding uplands are lowest, and also where a major tectonic system coincides with the edge of Sputnik Planitia.  The low elevation of the area makes it a natural setting for past coverage by nitrogen glaciation as indicated by modeling of volatile behavior on Pluto.  Through comparison of the washboard and fluted texture with fields of sublimation pits seen in southern Sputnik Planitia, the ridges are interpreted to represent refractory debris liberated by tectonism of the water ice crust.  This water ice debris was buoyant in the denser, pitted glacial nitrogen ice that formerly covered this area, and collected on the floors of the pits.  After the nitrogen ice receded via sublimation, the debris was left as the aligned ridges – washboard where deposited on flat terrain, and fluted where deposited on steeper slopes.  Crater surface age estimates indicate that this nitrogen ice glaciation formed and disappeared early in Pluto’s history, soon after formation of the Sputnik Planitia basin, to which surface nitrogen ice across Pluto migrated.  The precise mechanism that elongated the sublimation pits and defined their strikingly consistent orientation regardless of latitude or location relative to Sputnik Planitia is elusive, but is consistent with a global-scale process.  A constraint is that true polar wander solutions for Pluto indicate that the ridges can never have all been oriented parallel to lines of longitude at any time in Pluto’s history.  This suggests a cause for the alignment that is not exogenic (i.e. the orientation is likely not governed by solar illumination causing the sublimation pits to align N-S).  These terrains constitute an entirely new category of glacial landform.

 

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